The present invention relates to the field of overvoltage protection circuits.
Among overvoltage protection circuits, one knows, for example from U.S. Pat. No. 4,282,555, circuits as shown in FIG. 1 wherein three unidirectional protection units 1, 2 and 3 are connected between a common point C and a first line A, a second line B and ground M. Each protection component comprises a non-parallel unidirectional protection component P and a diode D, respectively P1, D1; P2, D2; P3, D3. The electrodes of same nature (anode or cathode) of the various components are connected at point C.
The protection components P1, P2, P3 are, for example, gateless thyristors which become conductive from their anode to their cathode when the voltage across their terminals exceeds a threshold value determined during manufacturing.
An advantage of the circuit of FIG. 1 is that the three units can be integrated on a single chip.
However, whenever the advantageous solution of monolithically integrating the three protection units is selected, as a result, for technical reasons, the breakdown voltages of the three protection units P1, P2, P3 are identical and will increase with the temperature of the unit, heating being liable to be caused by a long-time overvoltage or by short-time and recurrent overvoltages.
On the other hand, some circuits are entirely achieved as discrete circuits which are costly and cumbersome and are designed only for the protection of one phase, as described in French patent FR-A-70 17172.
Thus, an object of the invention is to provide a protection circuit which gets rid of the drawbacks of the fully integrated circuits, that is, wherein, on the one hand, the protection voltage of each component is adjustable independently of that of the other components and, on the other hand, the protection voltage is independent of temperature, and is not so costly as discrete circuits.